Field of the Invention
This invention relates to an improved process for the separation or removal of oil from waste water which contains the oil in the form of fine particles.
Description of the Prior Art
An oil which is dispersed in water in the form of fine particles particularly an oil which is converted into a stable emulsion by the action of an anionic surface active agent, is hard to separate by a conventional method using the specific gravity difference or a filtration or adsorption method. Conventionally, the dispersed oil is treated with a coagulation-separation method wherein aluminum sulfate, ferric chloride or equivalent compound is used as a coagulating agent. However, the coagulation-separation method presents the following difficulties. Firstly, a solid-liquid separating apparatus is required in order to separate the resultant scum. Conventionally, a pressure floating device has been used as the solid-liquid separating apparatus. This device, however, is of fairly complicated structure and costly, and requires a wide floor space. In addition, in order to operate stably with this device, meticulous care must be exercised in its maintenance. Secondly, since the scum separated by the above method usually contains 97 to 99% by weight of water, it is difficult to burn it off as such, and the scum must be dehydrated before the burning treatment. An Oliver filter, for example, is used for this purpose, but this device is as expensive as the pressure floating device and requires careful maintenance from the standpoint of the zig-zag movement, the clogging, and the washing of the filter cloth. Thirdly, the scum so dehydrated must be burnt off. This requires a large-sized equipment. Accordingly, there is a need for a method of coagulating oil alone without producing any scum.
Also exemplary of the prior art is a process disclosed in U.S. Pat. No. 3,347,786 Baer et al. which adds metal salts or metal oxides to the waste water and flocculates the same as their hydroxide by setting a suitable corresponding pH in the waste water and by repeated dissolvings and readjustments of the pH attempts to remove the emulsified oil. The salts added by Baer et al. are those of aluminum magnesium and iron. This process was not felt to be efficient enough and a search was therefore begun to achieve a process that gave better results.